It is known to treat exhaust gases from a lean burn internal combustion engine with a catalytic aftertreatment component comprising both platinum (Pt) and palladium (Pd). See for example WO 2004/025096.
It has been suggested to oxidise carbon monoxide (CO) in exhaust gas from stoichiometric exhaust gas—as opposed to lean-burn exhaust gas—to carbon dioxide (CO2) using a catalyst that comprises co-precipitated noble metal particles and metal oxide particles, such as Au/CeO2 (see EP 602865).
Furthermore, it has been suggested to catalytically convert CO emitted from a smoking device such as a cigarette to CO2 using a layered metal oxide catalyst comprising a plurality of metal oxide layers, wherein an outer layer may comprise one or more noble metals such as gold, silver, platinum, palladium, rhodium, ruthenium, osmium or iridium or a mixture thereof (see EP 0499402).
U.S. Pat. No. 4,048,096 discloses the use of palladium-gold alloys deposited on a catalyst support for the preparation of vinyl esters.
GB2444125A discloses an engine exhaust catalyst comprising a first supported catalyst and a second supported catalyst. The first supported catalyst may be a platinum catalyst, a platinum-palladium catalyst or a platinum catalyst promoted with bismuth. The second supported catalyst comprises palladium and gold species. The first and second supported catalysts are coated onto different layers, zones or substrate monoliths. In one arrangement an inner layer comprising the second supported catalyst is separated from an outer layer comprising the first supported catalyst by a buffer layer. The document does not mention Pd—Au alloys. Furthermore, it explains that the formation of less active Pt—Pd—Au ternary alloys should be avoided, hence the use of the buffer layer to separate the Pt from the Pd—Au.
WO 2008/088649 discloses an emission control catalyst comprising a supported platinum-based catalyst, and a supported palladium-gold catalyst. The two catalysts are coated onto different layers, zones or substrate monoliths such that the Pt-based catalyst encounters the exhaust stream before the palladium-gold catalyst. Similarly to GB2444125A, the document does not mention Pd—Au alloys, but explains that ternary Pt—Pd—Au alloys should be avoided.
There exist a number of difficulties in treating lean-burn exhaust gas to meet existing and future emission standards throughout the world, including Euro IV, V and VI in an efficacious and cost-effective manner. In the latter regard, it will be appreciated that the cost of platinum is presently over US$2000 per troy ounce. A number of particular difficulties include meeting emission standards for “tailpipe” hydrocarbons by oxidising unburned hydrocarbon fuel to CO2 and water; and that whilst there have been moves throughout the world to reduce the quantity of sulphur present in fuel (ultra low sulphur diesel (ULSD) available in US contains a maximum of 15 ppm sulphur and diesel containing 50 ppm sulphur is currently mandated in Europe, falling to 10 ppm from January 2009), sulphur poisoning of aftertreatment catalysts remains an issue, particularly as on-board diagnostics-based legislation is introduced.
Whilst use of palladium in combination with platinum has reduced the cost of catalytic aftertreatment components, the use of palladium in diesel oxidation catalysts is somewhat limited due to its relatively lower reactivity under very oxidising (lean) conditions relative to platinum. Unlike platinum, which has a higher ionisation potential and lower oxide stability, palladium exists mostly as an oxide with low specific activity for the oxidation of CO and hydrocarbons (alkene and long chain alkane). Furthermore, where passive regeneration of filters by combusting trapped particulate matter in nitrogen dioxide from oxidizing nitrogen monoxide present in exhaust gas is desired (according to the process disclosed in EP 0341832), palladium has a lower specific activity for NO oxidation under the high O2 concentration condition typical of lean burn exhaust, e.g. diesel.
Palladium is also known for its ability to readily react with sulphur dioxide (SO2) to form a stable sulphate. The decomposition of palladium sulphate in a lean environment requires temperatures in excess of 700° C., or lower temperatures (e.g. 500° C.) in rich exhaust gas but at a fuel penalty for creating the rich environment.